It is desirable in many applications involving the transmission and reception of microwave signals, to alter the direction of travel of a microwave signal by introducing a reflector into its path. In the case where the reflector is flat, the reflective surface acts in a manner analogous to a mirror in that an incident microwave signal is reflected in accordance with the law of optics. In designing curved reflecting surfaces which enable the concentration or focusing of incident microwaves, optical theory can be applied in a reliable manner, the reason being that microwaves, on a large scale, are propagated in straight lines and, like light waves, microwaves undergo reflection, refraction, diffraction, and polarization.
One particular example of applying optical theory in the design of curved reflecting surfaces, is found in the parabolic antenna. The theory of operation of the parabolic reflector antenna can be most easily explained by the use of ray tracing theory.
As illustrated in FIG. 1, if a microwave transmitter is placed at an infinite distance from a parabolic reflector, then the microwaves which reach the reflector are parallel. Due to the parabolic geometry of the reflecting surface, the parallel beam of microwave radiation is reflected through its focus. Conversely, since all reflection processes are reciprocal, the parabolic reflector will produce a parallel microwave beam if the source of microwave radiation is placed at its focus.
As in the case of the parabolic reflector where reflecting (i.e., focusing) all the incident microwaves towards a single point (i.e., focal point) is required, the reflecting surface must be properly curved. This process of focusing the microwaves, not only requires that the microwaves are reflected in the proper direction towards the focus point, but also that all the reflected microwaves arrive at the focus at the same time, which is commonly referred to as arriving or being "in phase".
In a reflector antenna, such as a parabolic reflector, proper "phasing" of the reflected microwaves is accomplished by ensuring that the distance travelled, or path length, of each incident microwave signal transmitted from the transmitter to the focal point, is identically the same. Where this criterion is not satisfied, "phase distortion" of the incident microwave signals occurs, posing serious reception problems in nearly all instances. In fact, this criterion is so essential that the equation defining the geometries of parabolic reflectors are often based on the criterion, calling for equalized path lengths. This concept is illustrated in FIG. 2.
In some instances where space limitations require that the shape of the parabolic reflector be altered from its characteristic geometry, several prior art techniques are known by which the path length of incident waves can be equalized to satisfy the above-mentioned path length criterion.
Utilizing a known optical design technique, the parabolic reflector of FIG. 2 can be emulated by using the antenna configuration of FIG. 3. Therein, a flat plate reflector is shown on which a dielectric "lens" is mounted in order to provide the desired path length compensation using the principle of refraction.
In the antenna configuration of FIG. 3, the overall thickness of the reflector-dielectric lens assembly is substantially similar to that of the parabolic reflector which it emulates, although the curvature of the dielectric lens is different.
One approach to reducing slightly the thickness of the dielectric lens employed in the prior art path length compensation technique, could involve the use of a Fresnel type lens which approximates the optical and geometrical characteristics of any particular dielectric lens. Methods for making such types of lenses can be found, for example, in U.S. Pat. Nos. 3,739,455 and 3,829,536 to Alvarez and 4,643,752 to Howard et al.
However, while the use of Fresnel lens can reduce slightly the thickness of dielectric lenses employed as path length compensation devices, the resulting microwave device suffers from serious drawbacks and shortcomings. In particular, the resulting surface of the dielectric lens is restricted primarily to planar surfaces and cannot conform to any arbitary surface, as would be desired. Also, manufacturing of such dielectric lens is time consuming and expensive, and the resulting surfaces are prone to collect undesirable airborne matter. In addition, the resulting structures lack the degree of ruggedness and durability required in many applications.
Thus, one of the major problems with such designs is that physical configuration of reflectors cannot be made substantially thinner than the curved reflector antenna configuration sought to be emulated using path length compensation techniques known in the art, and without the aforedescribed shortcomings and drawbacks.
It is desirable, therefore, to achieve reflection of microwave signals in a manner characteristic of curved reflector antennas while achieving the same using an antenna structure which is substantially thinner than curved reflector antennas sought to be emulated using path length compensation techniques (i.e., dielectric lens) known hitherto.
Moreover, it is desirable in some applications to achieve reflection of microwave signals in a manner characteristic of curved reflector antennas, using reflector antenna configurations that may be made to conform with other arbitrary curved surfaces, such as, for example, an airframe surface, and still provide a desired reflective surface of a selected geometry, e.g., a parabolic surface.
In some applications, it is also desirable to achieve focusing of microwave signals in a manner characteristic of curved refractive lens while achieving the same using an antenna structure which is substantially thinner than curved refractive lens sought to be emulated using known path length compensation techniques.
Accordingly, it is a primary object of the present invention to provide an electrically thin microwave phasing structure for electromagnetically emulating a desired reflective surface of selected geometry over an operating frequency band.
The desired reflective surface can be of any geometry, including parabolic surfaces, and geometry of the microwave phasing structure can be made to conform to any arbitrary surface, including planar surfaces.
It is another object of the present invention to provide such a microwave phasing structure, the overall thickness of which can be less than the fraction of the wavelength of the operating frequency of the microwave phasing structure.
It is a further object of the present invention to provide an electronically passive phase delay mechanism of an electrically thin configuration, mountable onto the surface of a reflector, which can be flat, for purposes of equalizing the path lengths of incident microwaves to a focal point, by providing an electronically introduced phase shift thereto as it is being reflected, in contrast with effecting path length compensation based on principles of refraction. The inventive concept of the present invention can be applied provided that Maxwell Equations are applicable.
A further object of the present invention is to provide a method for electromagnetically emulating a desired reflective surface of selected geometry over an operating frequency range, using an electrically thin microwave phasing structure.
It is a further object of the present invention to provide an electrically thin microwave phasing structure for electromagnetically emulating a desired microwave focusing element of selected geometry.
An even further object of the present invention is to provide a method of focusing electromagnetic waves using the microwave phasing structure of the present invention.
A further object of the present invention is to provide a method of shaping radio frequency (RF) energy which greatly increases the configuration flexibility of reflector antenna designs.
The concept of another object of the present invention, is to provide methods of manufacturing electrically thin microwave phasing structures for electromagnetically emulating desired reflective surfaces and focusing elements of selected geometry.
Other and further objects of the present invention will be explained hereinafter, and will be more particularly delineated in the appended claims, and other objects of the present invention will be apparent to one with ordinary skill in the art to which the present invention pertains.